Electrodeposits of tin, tin/lead, tin/antimony, tin/bismuth, and the like, are utilized in the finishing of electronic components. Most plating formulations employing tin salts rely on tin in the stannous (+2) oxidation state. The normal operation of tin alloy electroplating solutions can result in significant oxidation of stannous (+2) to stannic tin (+4). Stannic tin is not readily electropdeposited from methanesulfonic acid based plating baths, and the build-up of Sn(+4 or IV) in a plating bath is undesirable.
The tin oxidation process is not fully understood, but the involvement of active oxygen is suspected. Factors which lead to increased solution oxygen concentration are known to contribute to tin oxidation, though oxidation rate acceleration beyond that attained in an oxygen saturated solution is not normally observed.
The formation of stannic tin derived sludge is a particularly undesirable aspect of tin oxidation. In practice, tin and tin alloy electroplating baths are formulated with an antioxidant which prevents the oxidation of stannous to stannic tin. Hydroquinone, catechol, phenidone, morin hydrate, and vanadium (V) oxide are representative examples of known antioxidants. Many antioxidants, such as the dihydroxybenzenes, are believed to function by reacting with the active oxygen compound(s) responsible for tin oxidation. In the case of vanadium (V), a mechanism involving catalysis of the reaction between stannic and metallic tin has been proposed.
It is well known that different plating electrolytes, under a given set of otherwise identical conditions, strongly influence the extent of tin oxidation. Methanesuffonic acid (CH.sub.3 SO.sub.3 H) and fluoboric acid (HBF.sub.4) are two well known examples of conductive electrolytes for tin and tin alloy plating. Methanesulfonic acid based tin alloy plating solutions are relatively resistant to tin oxidation when compared to fluoboric acid based tin alloy plating solutions. The tendency for tin oxidation in fluoboric acid based systems is so great that an oxygen sparged fluoboric acid based tin electrolyte without an antioxidant will be completely converted to stannic tin within an hour. A methanesulfonic acid based tin electrolyte exposed to oxygen sparging without an antioxidant will typically take several days before 50% stannic tin is present. This marked difference between methanesulfonic acid based and fluoboric acid based electrolytes has been well known within the electroplating field for many years.
While antioxidant free methanesulfonic acid based electrolytes are significantly better than antioxidant free fluoboric acid based electrolytes for tin and tin alloy plating, it is still advantageous to incorporate an antioxidant into a methanesulfonic acid based system. It is further desirable that the antioxidant added to a tin alloy electroplating formulation have additional positive influences on the electrodeposition process. These can include grain refining, selective metal coordination, and selective precipitation of bothersome impurities.
Grain refining involves all chemical processes which influence the morphology and average size of the microscopic electrodeposit surface. The importance of grain morphology and size is well known in the art.
Selective metal coordination is used in alloy plating formulations where the different reduction potentials of the metals being deposited lead to selective enrichment of one metal over the other in the electrodeposit. The proper degree of selective coordination will result in a plating formulation which deposits a consistent alloy at different current densities. A consistent alloy is particularly important in the electronics industry, where deposit solderability and performance are critically affected by alloy.
The value of materials which selectively precipitate undesirable impurities is obvious. One problem associated with tin oxidation is the removal of the finely dispersed stannic tin which forms. Stannic tin at low concentrations is seen as a cloudy impurity which is difficult to filter free from plating solutions. Normally, flocculation prior to stannic tin filtration is necessary. Chemical agents which selectively precipitate stannic tin in a form which can be removed by commonly used filter media eliminate the need for time consuming flocculation treatments. Such selective stannic tin removal also reduces the potential deleterious effects of soluble stannic tin on electrodeposit quality.